Instead it seems more feasible that the corona is not powered by
its own mass accretion supply, as in the truncated disc models, but is
instead powered by mass accreted through the disc, but whose energy is
released in the corona (e.g. [1]}). However, this model
itself runs into difficulties since the simplest idea would be for
magnetic buoyancy to transport the energy vertically. The energy is
then released in a corona above the disc, so the corona is co-spatial
with it and illuminates it. This gives rise to a reprocessed
luminosity \(L_{rep}=\frac{1}{2} (1-a) L_c\)  (where \(a\)  is the albedo,
and the factor \(\frac{1}{2}\)  assumes the corona emits isotropically)
adds to the intrinsic disc luminosity [2]}. For hard spectra,
such as those observed here, the reflection albedo \(a<0.3\) , hence the
disc flux should be at least \(L_c/3\) . Yet we observe a disc flux of
\(\le L_c/20\)  towards the end of the outburst. Thus the only way to
circumvent the reprocessing limits whilst having all the mass accrete
via the disc is if the energy is advected radially as well as
vertically. Then it can be released in a more centrally concentrated
region, on size scales smaller than the (small!) disc inner
radius. The alternative of having the hard X-rays be strongly beamed
away from the disc, is ruled out by the very similar dim low/hard
state spectra seen in XTE J1118+480 ([3]}; 2003;
[4]}), which has high inclination (\(\ge 70^\circ \) ) so
that we must see a similar hard X-ray flux to that of the disc.
